The present invention relates to nuclear reactors and more particularly to support and guidance structure for control rods in such reactors.
In a typical pressurized water reactor for an electric power plant, over fifty elongated control rod assemblies are located in plenum space above the reactor core and disposed for vertical movement into or out of the core to control or shut down the nuclear reaction. Normally, only four or five control rods are needed for power generation control and the remaining rods remain fully withdrawn during reactor operation for use in shutting the reactor down when required or desired.
A control rod drive is provided for each control rod assembly above the reactor pressure vessel dome. Typically, a drive rod extends downwardly from each drive through a vessel dome penetration to a point within the reactor vessel where it is coupled to the control rod itself. In turn, the control rod is formed by a cluster of rodlets suspended from a spider assembly that is coupled to the drive rod.
Each control rod assembly is aligned with an individual fuel assembly within the reactor core. The rodlets in each control rod assembly are aligned for entry into respective vertical thimbles interspersed with vertical fuel rods in the fuel assembly. Fuel assemblies at preselected core locations are designated for control rod operation and the number and location of control rod assemblies are correspondingly determined.
A vertical guide tube structure houses each control rod including its rodlet cluster in space above the reactor core to provide guided support for the control rod against horizontal forces when it is fixed in its position or as it moves vertically toward or away from the reactor core.
Generally, the space above the upper core support plate within the vessel is divided by an upper support plate into an upper head plenum above the plate and an outlet plenum below the plate. Hot upward coolant flow from the reactor core is collected in the outlet plenum and directed through outlet nozzles to two or more steam generators. The number of outlet nozzles corresponds to the number of coolant loops provided for the reactor plant.
Cold coolant flow from the steam generators is returned to the reactor vessel through inlet nozzles to an annular downcomer space inside the pressure vessel wall where it is directed downwardly to the bottom of the core for another upward heat collecting pass through the core.
Spray nozzles are spaced peripherally about the vessel/dome flange to direct some of the return flow as a core bypass flow into the upper head plenum where it provides dome and control rod drive cooling. It returns to the outlet plenum through the control rod guide tube structures. Pressure differential between the downcomer and the upper head plenum determines the amount of coolant flow through the upper head spray nozzles.
In one plant type referred to as a "cold temperature" plant, the upper plenum coolant reservoir is held at the cooler inlet temperature by providing enough bypass flow into the upper plenum to disallow mixing of upward flowing core outlet flow into the upper plenum. The cold upper plenum water provides adequate core cooling capability in case of a possible loss of coolant accident in this plant type.
In a "hot temperature" plant, some coolant flows upwardly through some of the radially inwardly located control rod guide tube structures from the outlet plenum thereby providing a portion of the total inlet flow to the upper head plenum and a somewhat warmer upper plenum reservoir. Typically, a cold plant has an upper plenum temperature of 550 to 560.degree. F. whereas a hot plant has an upper plenum temperature of 590.degree. to 620.degree. F.
Each control rod guide tube structure normally includes an upper solid tube that is secured to the upper support plate and extends upwardly within the upper plenum. Further, a lower guide tube is secured to the upper support plate in alignment with the upper guide tube and extends downwardly within the outlet plenum. Horizontal cards are disposed within the guide tubes at spaced locations to provide control rodlet guidance support along the guide tube height. The lower guide tube usually has small wall openings along its height at the card locations through which coolant may flow from the guide tube into the outlet plenum, or vice versa in the case of some guide tubes in a hot plant.
While coolant flow is needed within the guide tubes for transmitting flow into or out of the upper plenum, it also contributes to control rod wear problems. Rodlet wear typically occurs in the vicinity of the horizontal rodlet support cards as a result of flow induced rodlet vibrations. The rodlet wear is usually significantly greater in the upper plenum region, and it varies in accordance with the radial position of the control rod assembly. Rodlet wear is reduced in hot plants as compared to cold plants, but it is a problem in both types of plants with differences occurring mainly in the time it takes for a given level of rodlet wear to occur.
The present invention is directed to controlling the flow around the control rods in the guide tubes to provide improved control rod operation with reduced rodlet wear.